The invention is particularly concerned with improving the mode locking performance of imperfectly mode locked lasers producing non-transform limited pulses. Short, transform limited pulses are achieved by ensuring that the lasing modes have a specific phase relationship to each other. If all the modes within the laser gain curve could be locked then the laser pulse width would only depend on the gain bandwidth. In most lasers the extent to which this can be achieved is limited by the physical properties of the mode locking element. In a Nd:YAG laser the limitation is the modulation depth and in F centre laser the limitation is the thermal destruction of the lasing medium.
In a paper entitled "The Soliton Laser" published in Optics Letters Vol 9, page 13, 1984, a method of improving the mode locking performance of lasers is described. This method consists in selecting a particular length of optical fibre to form a feedback loop. The idea behind this proposal was to reinject the light from the fibre into the laser cavity so as to force the laser to produce solitons in the optical fibre. The authors suggest that the reason why this improves a mode locked laser is due to the periodic nature of the solitons in their propagation along the fibre. This results in a stable operating point as the input and output pulses of the fibre are identical thus leaving the circulating pulses in the laser cavity undisturbed.
In a later paper by LF Mollenauer entitled "Solitons in Optical Fibres and the Soliton Laser" in Phil. Trans. R. Soc. Lond. A 315,437-450 (1985) it is indicated that soliton effects are possible only in the region of negative group velocity dispersion. This has a limiting effect on the wavelengths which can be produced using a soliton laser. The wavelength used typically is 1.55.mu.m. A further problem is in the difficulty of selecting an appropriate medium for the external cavity. The papers mentioned above both indicate that the non-linear and dispersive affects of the medium are equally important and it is necessary to select a medium in which these two effects exactly balance one another or at least nearly balance so that the frequency modifications imposed on the pulse by the two effects are of opposite sign and cancel each other out. This again leads to limitations in the type of media which can be chosen for the external cavity. Furthermore, very high peak powers are required in the fibre cavity.
In accordance with a first aspect of the present invention, a radiation generator in which substantially all the radiation modes are locked with respect to one another comprises a radiation generating cavity for generating a continuous train of non-transform limited pulses of coherent radiation; an external cavity positioned in association with the radiation generating cavity whereby radiation may pass in use in both directions between the two cavities, the external cavity being defined by a medium having a non-linear loss characteristic which predominates over any non-linear refractive index property of the medium, as hereinafter defined, for radiation from the generating cavity, and having a radiation path length such that pulses fed into the external cavity are fed back to the radiation generating cavity in synchronism with a later pulse from the generating cavity.
In accordance with a second aspect of the present invention, a radiation generator in which substantially all the radiation modes are locked with respect to one another comprises a radiation generating cavity for generating a continuous train of non-transform limited pulses of coherent radiation; an external cavity positioned in association with the radiation generating cavity whereby radiation may pass in use in both directions between the two cavities, the external cavity being defined by a medium which has a non-linear transmission characteristic, as hereinafter defined, for radiation from the generating cavity and a negative group delay dispersion, and which defines a radiation path length such that pulses fed into the external cavity are fed back to the radiation generating cavity in synchronism with a later pulse from the generating cavity.
In accordance with a third aspect of the present invention a radiation generator in which substantially all the radiation modes are locked with respect to one another comprises a radiation generating cavity for generating a continuous train of non-transform limited pulses of coherent radiation; an external cavity positioned in association with the radiation generating cavity whereby radiation may pass in use in both directions between the two cavities, the external cavity being defined by a medium which has a non-linear transmission characteristic, as hereinbefore defined, for radiation from the generating cavity and which does not support soliton propagation therein, and which defines a radiation pathlength such that pulses fed into the external cavity are fed back to the radiation generating cavity in synchronism with a later pulse from the generating cavity.
We have constructed a mathematical model of the soliton laser described in the paper mentioned above. This model reproduces the main features of the experiment and indicates, surprisingly, that the generation of solitons is not the reason why an improvement in mode locking is achieved. We have deduced an alternative explanation which is that it is the non-linearity in the external cavity which provides a means for the higher frequency modes to communicate and hence lock their phases together. The only role of the mode locking element in the laser cavity is to provide a timing for the pulses.
On the basis of this explanation, we predicted that the soliton nature of the fibre was unnecessary and that the important feature was a non-linear medium. Using our model, we replaced the optical fibre by another, simple non-linear element such as a saturable absorber and have shown that a similar improvement in mode locking performance is achieved.
This discovery leads to significant advantages being achieved over the previous proposal. It is not necessary to select a medium in which the non-linear and dispersive affects are approximately or exactly balanced. Instead, it is sufficient to consider the non-linearity of the medium, for example whether non-linear losses predominate over any non-linear refractive index or whether, if the medium has a predominant non-linear refractive index, it also has negative group delay dispersion effect. The removal of the requirement for soliton generation also means that a wider range of wavelengths may be generated and lower peak powers are possible. This leads to the possibility of using an external cavity to improve the mode locking of other lasers which cannot produce sufficient power for there to be a significant non-linear effect in an optical external cavity.
In this specification, by "non-linear transmission" we mean that the refractive index and/or loss of the external cavity is a function of the intensity of the injected radiation at a given frequency. In optical fibres this is essentially the Kerr effect in which the refractive index (and hence the phase velocity) changes proportionally to intensity. In other materials the loss changes with intensity. The effects of these non-linearities considered is on the shaping of the pulses.
By "dispersive" we mean that the group velocity is a function of wavelength or frequency.
Preferably, the medium defining the external cavity has a negative group delay dispersion (or a positive group velocity dispersion). This is useful since many materials have such a property and this enables a wider range of wavelengths to be generated. For example the radiation generating cavity could be provided by a Nd:YAG laser.
In other examples substantially non-dispersive media may be used for the external cavity such as saturable absorbers and saturable amplifiers. An example of a saturable absorber is a semiconductor laser operating below threshold. A saturable amplifier may operate on a four wave mixing process (which includes frequency doubling). For such media, the non-linearity affects the loss, rather than the refractive index.
An optical fibre could be used in an example according to the second or third aspects of the invention but with a length different from that needed to propagate solitons. In this case, the radiation injected into the optical fibre would need a high intensity.
Although the invention is particularly applicable for use with lasers, the invention is also applicable to radiation at non-optical wavelengths.
In this context, the term optical is intended to refer to that part of the electro-magnetic spectrum which is generally known as the visible region together with those parts of the infra-red and ultra-violet regions at each end of the visible region which are capable of being transmitted by dielectric optical waveguides such as optical fibres.
The external cavity may conveniently include a reflector positioned downstream of the medium so that radiation transmitted through the medium is reflected back into the cavity and back to the radiation generating cavity.
Alternatively, where an optical fibre is used to define the external cavity, this may be formed in a loop starting and terminating adjacent the generating cavity. It is preferable if monomode fibre is used and we believe it is also preferable for the fibre to be polarisation-preserving.